The present invention relates to a stepper motor exciting circuit. Stepper motors have been extensively employed as a source of rotary power for conveying a recording sheet. A facsimile system is an example where such motors are used. In a recording sheet conveying mechanism, in order to reduce the recording time, the recording sheet is preferably moved at high speed for lines in which no data is recorded. For this purpose, it is necessary to provide a stepper motor exciting circuit which can switch the stepper motor among a plurality of speeds.
FIG. 1 shows a conventional stepper motor exciting circuit of this type. In this circuit, first terminals of two-phase exciting coils 1 and 2 are connected through a resistor 3 to a power supply terminal 4, while the other terminals of the exciting coils are selectively grounded through respective switching transistors 5 and 6. Similarly, first terminals of two-phase exciting coils 7 and 8 are connected through a resistor 9 to the power supply terminal 4, and the other terminals thereof are selectively grounded through respective switching transistors 11 and 12. The bases of the switching transistors 5, 6, 11 and 12 are connected to corresponding terminals of a changeover circuit 13. Four output terminals of a first decoder 14 and four output terminals of a second decoder 15 are connected to the input side of the changeover circuit 13. A counter 17 counts pulses of a clock signal 16 and supplies a signal representing the count value to the decoders 14 and 15.
In accordance with the count value of the counter 17, the first decoder 14 outputs a signal for "two-phase" excitation, for instance, while the second decoder 15 outputs a signal for "W1-2 phase" excitation, for instance. The changeover circuit 13 provides the output signal of the first decoder 14 or the output signal of the second decoder 15 at the four output terminals thereof according to an excitation changeover signal 18. As a result, the excitation system of the stepper mtor can be switched between the "two-phase" excitation mode or to the "W1-2 phase" excitation mode to change the speed of rotation of the stepper motor.
This stepper motor exciting circuit is disadvantageous in that it is intricate, and accordingly expensive, because it is necessary to provide as many decoders as there are modes of excitation.